Myopia (or near-sightedness) has become the most pervasive visual disorder in the world. About twenty-five percent of people in industrialized countries in the Western world, and more than fifty percent of people in industrialized Asian countries require optical correction for myopia. With increasing educational demands, the prevalence of myopia is increasing steadily. Extensive reading by children and adolescents appears to cause progressive myopia. Since increasing educational demands have increased the prevalence of myopia, optical correction, such as eyeglasses, contact lenses, and refractive surgery for myopia is a major health care expense.
Myopia is due primarily to an elongation of the posterior pole of the eye during the school age years. Structures in this region of the eye tend to be stretched during development, and their integrity is compromised. This causes greater risk to the effects of ocular trauma, diabetes, macular degeneration, and other diseases. This means that myopia is also a major contributor to irreversible blindness.
Referring to FIG. 1, in a normal eye, the cornea 10 and lens 12 at the front of the eye focus an image of the visual world on the retinal receptors 14 at the back of the eye. At the retinal receptors the image begins to be processed and sent on to the brain as a complex neural signal. A myopic eye is too long, so that the image of most of the visual world is focused in front of the retina. Consequently, myopia is treated by weakening the optical power in the front of the eye so that the image is focused on the retina. This means that eyeglasses, contact lenses and refractive surgery are not treating the basic disorder, but are merely counteracting the effects of ocular elongation. Each of these treatments has its own problems, is expensive, and in no way reduces the likelihood that the myopic person will contract one of the blinding diseases which are secondary to myopia later in life.
Eyeglasses, contact lenses, and to a lesser extent, refractive surgery can accurately correct myopic defocus (often referred to as the spherical error of the eye) by placing as much of the focused image as possible on the retina. Some eyes have an aberration that creates a difference in optical power between one meridional orientation and another. This aberration is known as astigmatism and is correctable with eyeglasses (although eyeglasses cause visual distortion) and with specialty contact lenses (which may be uncomfortable).
At least thirty other “higher order” aberrations can be measured and quantified in the human eye. Each of these aberrations contributes a different type of degradation to the retinal image. These aberrations are usually measured in the laboratory with a complex optical instrument in which a laser beam is aimed at the retina of people who have their pupils dilated with drugs. However, such aberrations can now be measured in children and adolescents without the use of bright light or the need for pupillary dilation with drugs.
Traditional clinical correction of optical defocus places the average position of an image of the visual world on the retina. However, parts of that image may be in front of or behind the retina due to the refractive properties of the eye's aberrations. Thus, most of the image in a “perfectly” corrected eye may be significantly out of focus due to these aberrations.
A small number of people have myopia due to rare inherited diseases or, in old age, in conjunction with diabetic crystalline lens changes. More than ninety percent of the people with myopia, however, develop it during their school age years. It has been shown that this progressive myopia is clearly related to a genetic predisposition (Pacella et al., “Role of Genetic Factors in the Etiology of Juvenile-Onset Myopia Based on a Longitudinal Study of Refractive Error,” Optom. Vis. Sci. 76, 381-386, (1999)) and to an intensity of school work, especially reading.
Animal studies have shown conclusively that blurring the visual world by scattering an image through the use of eyelid closure or smoked or sandblasted eyeglass lenses leads to myopia. Similarly, defocusing the visual world with minus lenses induces myopic response. Both blurring (i.e., general image degradation) and myopigenic defocus affects myopigenesis more in some species than in others and more in some breeds than in others within the same species. This suggests that the myopic response to environmental influences is genetically dependent.
Epidemiological studies and other studies demonstrate the same type of environmental-genetic interaction